Development of InP solar cells on inexpensive Si wafers

Both PN and NP InP solar cells on Si wafers were made with AM0 one-sun efficiencies up to 11%. Cells were irradiated to an equivalent fluence of 4/spl times/10/sup 18/ 1 MeV electrons/cm/sup 2/ with alpha particles from an Am-241 source, losing only /spl sim/25% of their BOL output power. Although the BOL efficiency is about half that of GaAs/Ge cells, InP/Si cells have power outputs exceeding typical GaAs/Ge cells at very high fluences, and are intended for use in high radiation orbits where EOL power of GaAs/Ge cells is inadequate. SIMS data shows zinc diffusion from an InGaAs contact cap controls junction depth in PN InP/Si cells, while an epitaxial emitter layer controls the junction depth in NP cells. NP cells have much lower emitter sheet resistance, but require a tunnel junction near the InP/Si interface because of Si diffusing out from the wafer, doping the initial InP n-type.